1. Technical Field
The embodiments herein generally relate to electrical filtering technologies, and, more particularly, to electrical gain filtering and noise shaping technologies.
2. Description of the Related Art
Active filters are often realized using transconductance-C (gm-c) topologies or op-amp based resistor/capacitance (RC) topologies. There are many ways to implement higher order filters using these two techniques. However both techniques tend to suffer from a limited noise performance. This is because the active and passive components employed in both techniques are in the signal path. Thus, they directly add noise to the signal at all frequencies (no noise shaping is employed). Hence, to achieve an acceptable post down-conversion mixer low noise filter topology using those techniques leads to an unacceptable power and chip area penalties.
Furthermore, for best dynamic range performance gain and filtering should be interleaved. Moreover, for best linearity the out of band signals should be filtered first. However, for best noise performance, the signal needs to be amplified first before filtering. Hence, a fundamental trade-off exists between cascading filter and gain stages.
The disadvantages of these techniques are: (1) higher noise that prevents higher order filters from being used in low noise application (such as a post-mixer amplifier in a wireless integrated receiver); (2) high linearity demands on the amplifiers used, especially the amplifier preceding the first filter stage; (3) larger chip area and power consumption are required to achieve a high dynamic range; and (4) the filter circuitry is in the signal path and hence contributes to degrading noise, offset, and matching.
In many applications it is required to amplify a desired signal that occupies a specific frequency band while simultaneously attenuating all unwanted signals outside the desired signal band. A wireless system, in general, is one category of such a system. In integrated wireless receivers the desired signal is down-converted to the baseband frequency together with many unwanted blockers as shown in FIG. 1. The baseband section usually is required to amplify the unwanted signal, reject (filter out) the unwanted blockers, and demodulate the signal to recover the information. The problem facing integrated wireless receivers is that RF front-ends do not have any selectivity and hence the entire filtering operation should be performed at baseband. The RF section also can only provide a limited gain to the desired signal (due to the existence of blockers). Hence, the baseband filtering should add minimal noise to the signal. Accordingly, the traditional trade-off in receiver design arises. From a noise perspective, it is usually better to use amplifiers before filtering, however this places a big demand on the amplifier and the filter linearity spec (as shown in FIG. 2(A)). Another approach, as shown in FIG. 2(B), is to relax the linearity requirement by first filtering out the signal then amplifying it. However, this places a stringent noise requirement on the filter used. Hence, the overall dynamic range is limited either by linearity or by noise. Therefore, implementing higher dynamic range filters/amplifiers leads to more power consumption and larger die area. FIG. 2(C) shows a gain filter interleaved stages. This is an attempt to do some filtering followed by gain then more filtering followed by more gain and so on. This allows the linearity and noise to be traded off. Nevertheless, the first gain stage and first amplification stage in this topology are still going to be challenging. Furthermore, in all the configurations shown in FIGS. 2(A) through 2(C) the filter stages contribute to the overall offset and (I/Q) matching of the receiver. Therefore, there remains a need for a new gain filtering and noise shaping technique capable of minimizing the requisite chip area and power consumption levels.